In certain gas sensor applications, it is desirable to keep the sensor isolated from the external environment without impeding its functionality. Such isolation can be for the purpose of reducing or minimizing heat loss, reducing or minimizing the amount of light reaching the sensor, and/or reducing or minimizing the consequences of mechanical intrusion. Often, a sensor is operated at a given temperature, typically greater than that of the surrounding gas stream it is sensing. This is sometimes accomplished by the use of heat-producing devices disposed on the same substrate as the gas-sensing device. When this is the case, there is a finite amount of heat lost to the gas stream and structures surrounding the sensor. This heat loss is proportional to an amount of power loss from the entire system in which the sensor has been incorporated, and it is therefore desirable to reduce or minimize such heat loss. In addition, for some sensors it is desirable to limit the amount of light reaching the sensors. For most sensors, it is also desirable to limit mechanical or physical intrusion, either from particles entrained in the gas stream to be sensed or accidental occurrences such as the device being dropped.
Conventional, prior art thermal isolation techniques include fabricating the sensor itself in such a way as to create structures to provide thermal isolation (see, for example, U.S. Pat. Nos. 5,211,053, 5,464,966, 5,659,127, 5,883,009 and 6,202,467). Such exemplary thermal isolation techniques were designed specifically for the type of construction of the sensor involved and did not overcome the problems associated with heat loss at an assembly level, that is, where the sensor is configured as part of a greater assembly. Prior implementations of such gas-sensing devices, such as catalytically-based gas sensors, have employed different techniques to thermally isolate the device, such as suspending the device, within the gas stream being sensed, using individual wires that electrically connect the sensing device to its downstream processing and control circuitry (see, for example, U.S. Pat. No. 5,902,556), but these methods are not preferred for a sensor with multiple connections.
The foregoing prior art solutions have the disadvantage of being considerably more voluminous and bulky than is desirable for most end-uses. Additionally, design parameters of the prior art sensors have trade-offs, such as response-to-size and isolation-to-flow effects. “Response-to-size” refers to the relationship between the magnitude of the isolated gas volume exposed to a sensor and the amount of time for a complete exchange of the gaseous constituents within the isolated volume; the smaller the size of the isolated gas volume, the more rapid a complete exchange will occur. “Isolation-to-flow” refers to the relationship between the flow rate of a gas stream exposed to a sensor and the magnitude and rate of heat loss from the sensor; higher gas stream flow rates draw greater amounts of heat from a sensor more rapidly, thereby increasing power consumption by the sensor in order to restore lost heat. Moreover, such prior art solutions did not address the optical sensitivity of certain sensors. In this regard, capacitor-based devices are generally sensitive to light and can generate erroneous, stray signals upon exposure to light.